Observation of a threshold enhancement

in the p invariant mass spectrum

H.X. Yang(BES Collaboration)

IHEPyanghx@mail.ihep.ac.cn

Oct.28 - Nov.01, 2006, Guilin

Phys. Rev. Lett. 93, 112002 (2004)  Cited 28 times 

Outline

Introduction

Features of signal and event selection

Reliability of the result

Summary

IntroductionIntroduction

L ~ ~51030 /cm2s at J/ peak

Ecm~2-5 GeV

The Beijing Electron Positron Collider

VC: xy = 100 m TOF: T = 180 ps counter: r= 3 cm MDC: xy = 220 m BSC: E/E= 22 % z = 5.5 cm dE/dx= 8.5 % = 7.9 mr B field: 0.4 T p/p=1.78%(1+p2) z = 3.1 cm

BESII Detector

World J/ and (2S) Samples (106)

J/ (2S)

0

10

20

30

40

50

60

MarkI I I DM2 BES I BES I I

0

2

4

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8

10

12

14

MKI MKII MKIII CBAL BESI BESII

Motivation

The enhancement near the ppbar mass threshold observed by BESII in could be interpreted as a ppbar bound state (multi-quark meson).

The new observation of BESII as well as new resonances observed by other experiments makes the searches for multiquark states become a very hot topic of high energy physics field.

It is of special importance to search for possible baryon-antibaryon resonances or bound states.

,

,,, ppnppp

ppJ /

..ccpM ..ccpM

Data MC(Phase Space)

Data Data

pMpM

11

Strong enhancement near the threshold of ..ccpM

S-wave BW fit results

• M = (2075 +/- 12) MeV

Γ = (90 +/- 35) MeV

BR = (1.18+/- 0.28)*10**(-4)

• About 7σ statistical significance

Features of signal and event selection

Features of signal (I)

Signal means

• Λ signal (very strong constraint)

After selection, the clear Λ signal in data shows high purity of signal.

../ ccpKJ

Data MC

Features of signal (II)

• Four charged tracks, no photon, no pi0

• Three heavy particles: p, pbar, Kaon

• So kinematics (with correct mass assignment) provides very strong constraint in the event selection, and in the kinematic fit can reject most background events.

/JppK MM

0missP

Key points in event selection

Keep • High purity of signal ( Λ mass as a control)• Efficiency as high as possible• Systematic uncertainties from selection criteria

as small as possible

Low momentum feature should be considered

Event Selection

• Four good charged tracks ( Pxy>50 MeV ) • PID: Kaon-ID and at least one proton-ID• 4-C kinematic fit PID information is used in the mass assignment

• Remove events with combined information of Emiss and Mpk_miss

no requirement on Λ vertex reconstruction

..ccpK

High Purity of Signal after Selection

• It can be shown by the clean Λ signal

• MC background study: only 1~2%

Dominantly from

Data

..ccpK

Data/MC

Strong enhancement near the threshold of pm

Phase Space

Observed in both of and p

mpm

pM pM

pK Kp

S-wave BW fit results

•M = (2075 12 5) MeV

Γ = (90 35 9) MeV

BR = (1.18 0.28 0.4) 10-4

• M = (2044 17) MeV Γ = (20 45) MeV• 2/d.o.f = 32.5/26

P-wave BW fit results

The systematic errors arecarefully studied in S-wave case.

2/d.o.f = 31.1/26 ~ 7σ statistical significance high L hypotheses fail

Reliability of the result

Extensive and careful checks!

Similar enhancement also observed in pK'

Fix the parameters, 4 away from phase space.

Is the enhancement from N* or Λ*?

• Hard to understand why N* or Λ* only contribute to very small phase space

pKM KM

Other checks

• Can the enhancement be interpreted as known PDG particles? NO

• MC Input/Output test:

check systematic uncertainties of fitting bias – is small.

• Other fits (L=1, 2, 3): P-wave fine, other failed

• MC (phase space) also show non-uniform and asymmetric distribution of .

• The enhancement is consistent with S wave.

The distribution is consistent with S-Wave

pcos

)15.2( .. GeVM ccp pcos

Err: DataHis: MC

pcos

• 这一研究成果与新发现的质子－反质子奇异结构具有同样重要意义：有可能是一个多夸克态粒子（六夸克态）。

• 自然界已发现的由夸克组成的粒子中的夸克数均为两个或三个，因此发现（多于三个夸克组成的）多夸克态粒子对检验和发展量子色动力学 QCD 理论具有非常重要的意义。也将是新的物质存在形式。

• 两个 PRL Referee 均对该文章给予好评。（文章被直接接收）

• 被高能所作为重要科研成果对外宣传

重要科学意义

PRL Referee 给予好评• Referee A: …

由于 BES 等实验的多个新发现的共振峰，以及关于五夸克态粒子的争论，使得存在重子－反重子束缚态及多夸克态粒子的问题再次引起极大关注。…

• Referee B:

这篇文章研究了关于重子－反重子质量阈值共振结构的重要课题…。 BES 是第一个报道质子－反质子阈值增长结构的实验。而这篇文章观测到类似的 质量阈值增长结构，支持了前面的发现结果。…

p

Summary• We observed an enhancement at 2.08 GeV in the invariant

mass spectrum of

• Various checks show that the result is robust.

• It has been fitted with S-wave B-W function successfully:

mass = 2075+/-12+/-5 MeV

width=90+/35+/-9 MeV

Br=(1.18+/0.28+/-0.40)x10E-4

• It is very unlikely that the enhancement is from pure N* and Λ* interference, although they contribute to Dalitz plot.

• A possible multi-quark state candidate

p

THANK YOU!

• This is due to acceptance• It can be shown in distribution, where is

the decay angle of p in

Why Dalitz plot not uniform for events GeVM ccp 15.2..

pcos

p

..ccp

MCDATA

• W/o any constrains, PWA with N* and Λ* can fit data:

(S= -997)

• However, it needs many unexpected big BRs and many large destructive interferences to cancel these big BRs.

• Keep in mind: Ndata=5433

Mathematical fit

Fraction of Ndata Nevent

N*(1900) 3/2+ 108% 5900

N*(2050) 3/2+ 33% 1800

Λ*(1890) 3/2+ 21% 1100

Λ*(1810) 1/2+ 9% 500

Λ*(1800) 1/2- 34% 1900

(1/2- is P-wave decay, which should be suppressed )

Big BRs

• Typically, BR(J/psibaryon anti-baryon) ~ 2x10-3

e.g., BR(J/psi p pbar) =2.2x10-3

• So, if there is no suppression, the typical number of observed events for one decay mode is:

BR(J/psi Λ Λ *)BR(Λ *pK) BR(Λpπ) ε N J/psi

= 2x10-3 x0.1 x0.1 x0.6 x5.8x107

~ 700 events

Physics constrains (I)

• Phase space ~ p

• Angular momentum barrier

~ (p / M J/psi) 2L+1

• If the width is large, some fraction of N* or Λ* cannot be produced.

• Global efficiency may be lower.

Physics constrains (II) – near threshold suppression

• E.g., Since Λ* decays both to pK and Σπ,

and most BRs of them are of the same order, so this channel can provide constrains on Λ*(1520), Λ*(1600), Λ*(1690) production rate.

• No clear evidence for strong production of Λ*(1800), Λ*(1810), Λ*(1890).

Near threshold suppression is a reasonable estimation.

Physics constrains (III) – from other channels

Nevent/2 (each decay mode, not include c.c.)

N*(1900) 3/2+ ~ 300 -400 N*(2050) 3/2+ ~ 100 -150 Λ*(1890) 3/2+ ~ 100 -150 Λ*(1810) 1/2+ ~ 150 -200 Λ*(1800) 1/2- ~ 20 - 30 (1/2- is P-wave decay, which should be suppressed near threshold)

Estimated Nevent in Ndata

• Constrain the Nevent of near threshold states ~ 100-200

• The PWA fit cannot reproduce the enhancement near pΛ threshold (S=-900).

PWA with constrains and w/o X(2075)

• Adding X(2075), PWA fit can reproduce the enhancement near pΛ threshold.

• Significance of X(2075) >> 5 sigma. (S=-952)

PWA with constrains and with X(2075)

Interference of excited baryons?

• PWA fits with pure N* and Λ* can hardly reproduce the enhancement. (with reasonable constrains production rate for excited baryons)

• PWA fit with X(2075) can easily reproduce the enhancement with high significance. (independent of constrains)

It is unlikely that the enhancement is purely from * and N* interference.

• Fit the similar enhancement in with the same

parameters of X(2075) as in J/psi.

• The significance of X(2075) in psi’ data is ~4.0 σ.

• The similar enhancements in the psi’ data and in the Belle data ( in BpΛπ process ) CANNOT be from N*(2050), Λ*(1890) … interference.

Fit the enhancement in pK'

pK'

Summary of PWA studies• Reproducing the pΛ threshold enhancement with pure N* and

Λ* interferences needs many unexpected big BRs and large destructive interferences.

• PWA fits with pure N* and Λ* and with constrains can hardly reproduce the enhancement.

• PWA fit with X(2075) can easily reproduce the enhancement (independent of constrains) with high significance.

• The similar enhancements in the psi’ data and in the Belle data ( in BpΛπ process ) CANNOT be from N*(2050), Λ*(1890) … interference.

Systematic uncertainties

被高能所作为重要科研成果对外宣传